To maximize reaction kinetics, ceramic membrane reactors typically are designed to minimize membrane thickness. Reactors are designed with a thin film membrane layer supported by a thicker porous layer that also provides a path for reactant transport to the membrane. The support may be active, meaning it provides electrical conductivity or contains catalytic materials, or passive, meaning it provides only mechanical support. These supported membranes may be prepared by many methods, including chemical and electrochemical vapor deposition, sol-gel coating methods, spray and dip coating of particulate slurries, calendaring of multilayer samples, and screen printing.
Perhaps the most widely used process for producing supported membranes is tape casting, in which a porous-dense bi-layer is fabricated by the lamination of preceramic sheets containing the selected oxide powders, polymeric binders that provide plasticity to the tape, and a pyrolyzable fugitive phase that is incorporated into the support layer to prevent densification. The sheets typically are compressed at temperatures less than 100° C. to produce a laminate structure. The laminate is heated to ˜600° C. to remove the binder and fugitive phase, then fired to densify the membrane layer. Tape casting is material insensitive, and supported thin films of membrane material produced by tape casting have been deposited on cathodes, anodes, and inactive substances.
Conventional ceramic electrochemical cell designs are asymmetric in nature. The cells are supported by a thicker electrode, either the anode or the cathode, onto which the thin film is laminated, cast, printed, sprayed, or otherwise deposited. These cell architectures typically experience significant warpage during sintering because the layers sinter differently, with sintering behavior typically being dominated by the thicker electrode. To remediate sintering warpage, a weight may be applied to flatten the cell by creep mechanisms. Alternatively, weights can be applied to the cell during sintering to prevent deformation. Neither approach is desirable. The stresses imposed by the weight(s) typically create defects in the support, the electrolyte, or both.
After the sintering of the electrode-electrolyte bi-layer, the opposite electrode is typically applied to the electrolyte layer. For anode supported cells, this is necessary because the high processing temperatures required to co-sinter the anode and electrolyte preclude the application and sintering of lanthanum perovskite cathodes, which will react to form insulating lanthanum zirconate phases. For cathode supported cells, anode materials typically are applied separately because the lower temperature of the co-sintering process makes NiO/stabilized zirconia electrolytes difficult to apply. Nickel metal anodes are often used instead, being applied by plasma spray or dip coating and firing in a reducing atmosphere. In both cases, the extra sintering or other process steps add significantly to the cost of the cell.
Electrolyte/electrode interfacial resistance is a major contributor to the electrochemical cell resistance. Composite electrode materials often used to reduce interfacial resistance. Typical composite electrodes consist of an electronically conducting component and an ionically conducting component (often the electrolyte material). The mixed phases sinter to form interpenetrating networks of electronic and ionic conduction paths intertwined with porosity to allow gas diffusion throughout the structure.
A predominant mechanism for reduced interfacial resistance in composite electrodes includes an increase in the three-phase-boundary (tpb) area (i.e. where the electronic conductor, ionic conductor, and the gaseous environment meet the electrolyte). The interpenetrating network of ionic conductor serves as an extension of the electrolyte surface while the network of electronic conductor serves as an extension of the current collector phase. The ionically conducting path must be connected to the electrolyte layer and the electronically conducting path must be connected to the current collector for optimum operation. It therefore is critical that the ionically conducting material be strongly bound to the electrolyte layer of the composite electrode to provide useful area. If this is not the case, the electrochemical reactions are isolated and cannot enhance the transfer of charge.
Achieving adherence between the electrolyte layer and the tonically conducting phase of the composite can require high temperatures that result in detrimental chemical interactions between the electronically conducting phase and the electrolyte. In the nickel oxide-stabilized zirconia anode system, the high co-sintering temperatures required to bond the stabilized zirconia component to the electrolyte layer is benign because there is little interaction or solid solubility between NiO and stabilized ZrO2. However, it is difficult to achieve similar adherence between electrolyte and composite cathodes because of the interaction between stabilized zirconia and perovskite cathodes at high sintering temperatures. In the disclosed invention, this interaction is avoided by the sequential development of a porous electrolyte network that is strongly bound to the electrolyte layer and the subsequent infiltration of electronically conducting phases and precursors to form an electronically conducting phase.